1. Field of the Invention
This invention generally relates to borehole telemetry. More particularly, the invention relates to an electrically insulating gap sub assembly used for electromagnetic telemetry between surface and subsurface locations or between multiple subsurface locations.
2. Description of the Related Art
During a typical drilling operation, a wellbore is formed by rotating a drill bit attached at an end of a drill string. To provide for a more efficient drilling operation, various techniques may be employed to evaluate subsurface formations, such as telemetry, as the wellbore is formed. Generally, telemetry is a system for converting the measurements recorded by a wireline or measurements-while-drilling (MWD) tool into a suitable form for transmission to the surface. In the case of wireline logging, the measurements are converted into electronic pulses or analog signals that are sent up the cable. In the case of MWD, they are usually converted into an amplitude or frequency-modulated pattern of mud pulses. Some MWD tools use wirelines run inside the drill pipe. Others use wireless telemetry in which signals are sent as electromagnetic waves through the Earth. Wireless telemetry is also used downhole to send signals from one part of a MWD tool to another. The most commonly used drilling telemetry methods can be arranged into several distinct groups such as wireline, mud pulse, or electromagnetic (EM).
In the first telemetry group, wireline communication involves one or more insulated cables that has a wide bandwidth and thus can communicate large amounts of data quickly, but the cable must be pulled out of the hole when adding additional sections of drill pipe. This is time consuming and reduces overall drilling efficiency. It also may not be possible to rotate the drill string with the wireline cable in the hole.
In the second telemetry group, mud pulse telemetry, the drilling fluid is utilized as the transmission medium. As the drilling fluid is circulated in the wellbore, the flow of the drilling fluid is repeatedly interrupted to generate a varying pressure wave in the drilling fluid as a function of the downhole measured data. A drawback of the mud pulse technique is that the data transmission rates are very slow. Transmission rates are limited by poor pulse resolution as pressure pulses attenuate along the borehole and by the velocity of sound within the drilling mud. Further, while mud pulse systems work well with incompressible drilling fluids such as a water-based or an oil-based mud, mud pulse systems do not work well with gasified fluids or gases typically used in underbalanced drilling.
In the third telemetry group, electromagnetic (EM) telemetry, relatively low frequency (4-12 Hz) electromagnetic waves are transmitted through the earth to the surface where the signal is amplified, filtered, and decoded. Communication may also be accomplished in the reverse direction.
In a typical EM operation, generating and receiving the electromagnetic waves downhole involves creating an electrical break between an upper section and a lower section of a drill string to form a large antenna. Thereafter, sections of this antenna are energized with opposite electrical polarity often using a modulated carrier wave that contains digital information. The resulting EM wave travels through the earth to the surface where a potential difference may be measured between a rig structure and a point on the surface of the earth at a predetermined distance away from the rig.
Typically, the electrical break in the drill string is accomplished by a device referred to as a gap sub assembly. Generally, the gap sub assembly must electrically insulate the upper and lower sections of the drill string and yet be structurally capable of carrying high torsional, tensile, compressive, and bending loads. The known gap sub assembly includes an external non-conductive section with composite coatings to isolate the upper and lower sections. However, these coatings generally lack sufficient abrasion resistance when in contact with the abrasive rock cuttings and require frequent maintenance or replacement. In addition, the composite coatings typically do not provide a significant beneficial effect to the bending or compressive strength of the design. Additionally, the known gap sub assembly is expensive to manufacture. Furthermore, the known gap sub assembly is bulky and cumbersome to employ during a drilling operation.
Therefore, a need exists for a gap sub assembly that is capable of withstanding the abrasive environment of a wellbore. Further, there is a need for a gap sub assembly that is capable of withstanding the bending and compressive loading that occurs during a drilling operation. Furthermore, there is a need for a gap sub assembly that is cost effective to manufacture. Further yet, a need exists for a gap sub assembly that is compact and may be easily employed during a drilling operation.